The present invention relates to a process for the production of polyurethane moldings from a reaction mixture via the RIM process.
The production of polyurethane moldings via the reaction injection molding (i.e. RIM) technique is well known and described in, for example, U.S. Pat. No. 4,218,543. The RIM process involves a technique of filling the mold by which highly reactive, liquid starling components are injected into the mold within a very short time by means of a high output, high pressure dosing apparatus after they have been mixed in so-called "positively controlled mixing heads".
In the production of polyurethane moldings via the RIM process, the reaction mixture generally comprises an A-side based on polyisocyanates and a B-side based on organic compounds containing isocyanate-reactive hydrogen atoms, in addition to suitable chain extenders, catalysts, blowing agents, and other additives. The polyisocyanates which are suitable for a commercial RIM process are the aromatic isocyanates, such as, for example, diphenyl methane-4,4'-diisocyanate (i.e. MDI) and toluene diisocyanate (i.e. TDI).
Aliphatic isocyanates are broadly disclosed in the long list of suitable isocyanates for use in a RIM process in various patents, including, for example, U.S. Pat. No. 4,937,366. High productivity commercial RIM processes require a 30 s demold time, and prefer mold temperatures less than 80.degree. C. for worker safety and energy efficiency.
U.S. Pat. No. 4,772,639 describes a process for the production of polyurethane moldings reacting organic polyisocyanates with organic compounds containing isocyanate-reactive hydrogen atoms in the presence of catalysts and auxiliary agents inside a closed mold. The isocyanate component is based on (a1) mixtures of (i) 1-isocyanate-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (i.e. IPDI), and (ii) polyisocyanates containing isocyanurate groups prepared by the trimerization of a portion of the isocyanate groups of 1,6-diisocyanatohexane, or (a2) (i) IPDI and (iii) polyisocyanates containing isocyanurate groups prepared by the trimerization of a portion of the isocyanate groups of a mixture of 1,6-diisocyanatohexane and IPDI. These reaction mixtures are broadly disclosed as being suitable for RIM processing. However, the reference requires unusually long demold times, i.e. from 3-10 minutes. These demold times are not commercially acceptable.
IPDI (i.e., 1-isocyanate-3,3,5-trimethyl-5-isocyanatomethyicyclohexane) is a sterically hindered tertiary cycloaliphatic isocyanate. In general, this isocyanate tends to slow down the reactivity of the entire system, which makes it less desirable to use commercially. Higher mold temperatures and/or longer demold times are generally necessary for systems based on IPDI. U.S. Pat. No. 4,772,639 clearly requires that at least 10% by weight of the IPDI monomer be used.
U.S. Pat. No. 4,642,320 discloses a process for the preparation of a molded polymer comprising reacting inside a closed mold a reaction mixture comprising (a) an active hydrogen containing material comprising a primary or secondary amine terminated polyether having an average equivalent weight of at least 500, (b) at least one chain extender, and (c) an aliphatic polyisocyanate, polyisothiocyanate, or mixture thereof, wherein the NCX index is from about 0.6 to 1.5. This process requires that component (a) have at least 25%, and preferably 50% of its active hydrogen atoms present in the form of amine hydrogens. All of the examples disclose a system based on a HDI prepolymer with amine terminated polyethers and diethyltoluene diamine at high mold temperatures and long demold times. The isocyanate used in the examples is difunctional, and contains at least 39% by weight of HDI monomer. Although the reference discloses aliphatic isocyanates are suitable for this process, the mold temperatures are higher than normal, i.e. at least 90.degree. C., and the demold times range from about 1-5 minutes.
U.S. Pat. No. 4,764,543 discloses aliphatic RIM systems with short demold times (.about.10 seconds) and low mold temperatures (.about.70.degree. C.) that use very fast reacting aliphatic polyamines. This patent is restricted to total polyurea systems based on chain extenders which are cycloaliphatic diamines and polyethers which are amine-terminated polyethers. All of the working examples of this patent use methylene bis(4-cyclohexylisocyanate) which is difunctional, and 100% monomer. The present invention is directed to a method of obtaining fast reacting RIM systems based on urethane/urea technology with slower reacting OH terminated soft segments and a mixed extender package consisting of alcohols and amines. This is surprising to one of ordinary skill in the art, especially when used with the slow reacting aliphatic isocyanates. In addition, the invention requires the use of higher functionality polyisocyanates which are widely regarded as unacceptable building blocks for RIM elastomers as they traditionally lead to unacceptable tear and tensile strengths.
The present invention has several advantages over these references. Amine terminated polyethers (i.e. ATPE's) are very expensive. Accordingly, the cost of RIM systems which are based primarily on ATPE's is significantly more than the cost of RIM systems which use conventional hydroxy group containing materials. The present systems are more economical in terms of cost as they use smaller quantities, or none, of amine-terminated polyethers. It is also possible to adjust the catalyst level of the present systems to fit the specific processing needs of various applications. In addition, certain isocyanates are difficult to handle and may pose serious health concerns to workers. in certain preferred embodiments of the presently claimed invention, the low monomer content of the polyisocyanate significantly decreases the health concerns and risks associated with handling polyisocyanates.